Capacitor substrates made of refractory metal nitrides

ABSTRACT

Electrolytic capacitor powder substrates are provided of refractory metal nitrides to reduce instability at a substrate-oxide (as formed) interface whereby the resultant capacitor sensitivity to hear, bias and frequency is reduced.

FIELD AND BACKGROUND OF THE INVENTION

[0001] The present invention relates to refractory metal nitride powdersparticularly those of Ta, Nb and more particularly to such powder asusable in solid electrolyte capacitors.

[0002] Solid tantalum capacitors are built on tantalum metal substrateswith a dielectric layer composed of anodic tantalum oxide. A well-knownproblem with this structure is instability caused by oxygen migrationfrom the oxide to the metal substrate during thermal cycling (ref. 1). Agradient of oxygen deficiency with an associated conductivity profile iscreated across the anodic oxide film.

[0003] The present invention focuses on capacitors using a porous massof sintered nitrides (particularly TaN, Ta₂N, NbN) as the anode andnitride powders suitable for making them. The powders are referred to as“substrates” in relation to thin conversion “coatings” of dielectricoxide formed at the surfaces of the sintered powders of an anode slug(of various geometric forms) when contacted by a wet electrolyte underelectro-formation conditions. In the final product the pores of the slugcan be filled with a wet or solid electrolyte, but the invention isspecially beneficially for solid electrolyte capacitors. Capacitorscontaining a conductivity profile have high bias, frequency andtemperature dependence of capacitance. Although capacitor manufacturershave developed procedures to minimize or eliminate the oxygen deficiencyand associated conductivity profile in the dielectric, the thermodynamicinstability at the metal-metal oxide interface can contribute toignition and other failures of solid electrolyte tantalum capacitors.

[0004] Work over the last 30 years (ref. 2 and references therein) hasdemonstrated that anodic oxide films grown on tantalum substrates dopedwith nitrogen are more stable to thermal cycling. If the nitrogencontent in the tantalum exceeds 10 at, %; oxygen migration across thetantalum-tantalum oxide interface is suppressed. Capacitors made using anitrogen doped tantalum substrate are significantly less sensitive tothe effects of thermal cycling. In most cases, the substrates were thinfilms produced by sputtering tantalum in a nitrogen atmosphere.Sputtered substrate capacitors are found as micro devices in integratedcircuits. The sputtered capacitors made using a Ta₂N (33 at. % N)substrate were especially stable.

SUMMARY OF THE INVENTION

[0005] There is good potential for using TaN as the substrate for makinga bilayer suitable for making a solid capacitor. The anodic film grownon this substrate is insensitive to the effects of heat treatmentbecause the presence of nitrogen blocks the migration of oxygen acrossthe substrate-anodic film interface. The experiments show solidcapacitors made of powder with the TaN anodic film system can havenegligible bias, frequency and temperature dependence of capacitance andbe less susceptible to failure during long term aging. This was not madeavailable to the art prior to the last 20 years (see Ref. 3 [copyappended] and references cited therein).

[0006] The use of nitrogen to improve the performance of tantalumcapacitors made using tantalum as the substrate is known. U.S. Pat. No.5,948,447, granted Sep. 5, 1995 to H. Chang/Cabot Corp., describesnitrogen doping (at levels of 500-7000 ppm) of tantalum or niobiumpowder substrates to reduce leakage and speculating a beneficial effectin limiting crystal growth during anodization and the benefit of highersolubility of nitrogen in tantalum or niobium (compared to oxygen) tolower leakage by limiting movement of oxygen and a synergistic effect ofcontrolled combinations of nitrogen and oxygen content of the tantalumor niobium substrate. T. Tripp et al/H.C. Starck, Inc. in a symposiumpaper have described a 30 year long effort to investigate the effects ofnitrogen doping on tantalum substrates, mostly as to sputter depositedlayers of tantalum or tantalum nitride but including also nitrogen dopedpowder and describe current work that verifies the effect of nitrogen inretarding migration of oxygen across the metal (Ta or Nb)-anodic oxideinterface. D. J. Werder et al/Bell Telephone Labs (Thin Solid Films 323(1998): 6-9 provide transmission electron microscope images showing atantalum pentoxide anodic film formed on a sputter deposited TaNsubstrate with nitrogen rich inclusions in the lower (toward thesubstrate) portion of the oxide layer that appear to be associated witha decrease of dielectric constant.

[0007] Most examples of this technique involve thin film nitrogensubstrates prepared by sputtering tantalum in a nitrogen atmosphere.Niobium nitride powders are also disclosed for usage as substrates inelectrolytic capacitors in the published PCT application WO 98/38660(PCT/JP98/00823 filed Feb. 27, 1998 by K. Naito, Y. Uchida/Showa DenkoKK), in an overall process and product system involving nitriding aniobium powder (made by chemical reduction of a niobium fluorideprecursor) to produce niobium nitride powder, sintering the powder,oxidizing to form a niobium pentoxide layer (or forming such a layer insitu by a chemical vapor deposition process from an external precursor)as the dielectric, incorporating a variety of electrolytes in the poresof the sintered compact but preferably organic semiconductor systems andadding a cathode and packaging to define the complete electrolyticcapacitor.

[0008] The invention also includes a niobium powder that is deriveddirectly from a pure niobium pentoxide (Nb₂O₅), e.g. by reduction of thepentoxide with a magnesium vapor to form a niobium powder of extremelylow oxygen impurity content and no pentoxide content at all, thenintroducing nitrogen in a reactor schedule that precludes re-oxidationof the niobium—the schedule having multiple stages of thermal processingand environmental control defined below to establish a niobium nitridepowder compound without excess of nitrogen remaining and eventuallycooling under inert atmosphere and air release of the powder to formonly a limited oxide at room temperature.

[0009] Another objective is a substrate that provides for a morethermodynamically stable substrate-anodic film interface making thesystem less stable to the degradation that occurs in the niobium-niobiumoxide system, (and even in the tantalum-tantalum oxide system) duringthermal cycling.

[0010] Other objects, features and advantages of the invention will beapparent form the following description of preferred embodimentsthereof, including illustrative non-limiting examples, of the practiceof the process and the resulting products' configuration, compositionand performance under test conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIGS. 1-9 are graphs and profiles of test results and sampleconditions as described in Example 7 below.

[0012] A possible way of utilizing the ability of nitrogen to stabilizethe anodic oxide films formed on Ta or Nb powders and use these to makediscrete capacitors is to use a tantalum or niobium nitride (Ta₂N [33at. % N], TaN [50 at. % N] NbN [50 at. % N]) as the substrate. Thosecompounds can be pressed into pellets and processed using proceduressimilar to those in place for making solid tantalum capacitors and thereis potential for making high reliability, solid capacitors with goodvolumetric efficiency using such nitrides.

[0013] Similar considerations apply to other refractory metal nitrides(Ti, Zr, Hf) and their uses as capacitors and other electrode forms andas catalysts, filters and for medical purposes.

EXAMPLES

[0014] The invention is now further disclosed with reference to thefollowing non-limiting Examples.

Example 1

[0015] 1A. Experimental Methods

[0016] Tantalum nitride was prepared by heating 2.3 Kg of tantalumpowder in three temperature stages of 700 deg., 850 deg. and 1000 deg.C. The nitrogen was introduced in the form of N₂ gas mixed with argon.The initial gas mixture was 20-mole % nitrogen. At each temperature, thepowder was stepwise exposed to higher nitrogen gas concentrations with afinal exposure to pure nitrogen at 1000 deg. C. for 30 minutes. Theresulting black powder was crushed and screened.

[0017] The starting material was primary powder taken from a sodiumreduction run. The physical and chemical properties of this powder arelisted in Table I, below, along with those of the tantalum nitrideproduced. The nitrogen concentration of 6.43 W/W % translates to acompound composition of TaN_(0.90). (When the oxygen content is takeninto account, the compound is TaN_(0.9)O_(0.1).)

[0018] Pellets with a mass of 0.14 gm were pressed to a density of 5.5g/cc and sintered at 1500 deg. C. and 1600 deg. C. for 20 minutes. Thelead wire was tantalum. The pellets were anodized in 0.1 V/V % H₃PO₄using a current density of 100 mA/gm and a hold time at the formationvoltage of 2 hours. The formation temperatures were 25 deg., 60 deg., 80deg., 90 deg., 95 deg. C. Formation voltages were 16 and 50 volts.

[0019] The capacitance was measured using a HP 4262A LCR bridge withoutor with applied anodic bias. The maximum bias voltage was 9 volts forthe 16 V formations and 25 V for the 50 V formations. Anodized pelletswere heat treated in air at 400 deg. C. for 30 minutes. Reformationswere done in 0.1 V/V % H₃PO₄ held at the anodization temperatures. Thereformation time was 2 hours.

[0020] 1B. Results and Discussion

[0021] Table II, below, summarizes capacitance and leakage results as afunction of formation voltage, formation temperature and pelletsintering temperature. Depending upon the conditions, the capacitancevaried from a low of 23,000 to a high of 44,000 μFV/gm. The D.C. leakage(DCL) was high for pellets formed at 50 volts. Pellets formed to 16volts at 90 deg. C. and 95 deg. C. had acceptably low leakages.

[0022] Table III, below, lists capacitances and leakages after heattreatment. With one exception, the capacitance dropped on average about3%. This is in contrast to the behavior of the Ta-Ta₂O₅ system whichundergoes a 7-12% increase in capacitance after heat treatment. The DCLwas lower after the thermal cycle in most cases.

[0023] The heat-treated pellets anodized at 90 deg. C. were reformed atthe formation voltage for two hours at the formation temperature. Thecapacitance and leakage results after reformation are summarized inTable IV. With one exception the capacitance relative to the original orheat-treated value decreased 2-3%. The leakages of all four sets ofpellets were low and very low for the pellets sintered at 1600 deg. C.and formed to 16 volts. Clearly, the heat treatment/reformation processhad a very positive impact on the electrical quality as measured by theDCL.

[0024] Table V, below, summarizes the bias dependence of capacitance(BDC) after the various treatments. The BDC was taken as the percentchange in capacitance between the without bias value and the highestbias value. Of major significance, is the lack of a BDC after heattreatment. When the anodic film is grown on a tantalum substrate, thereis a 5-10% bias dependence of capacitance after heat treatment at 400deg. C. Of special interest is the fact that there was no BDC for thepellets that were reformed.

[0025] The lack of a bias dependence of capacitance after heat treatmentis strong evidence that a conductive profile was not generated in theanodic film. This is expected if oxygen is not extracted from the anodicfilm by the substrate and is consistent with the numerous earlierobservations that anodic oxide films grown on nitrogen doped substratesare resistant to oxygen migration across the metal-metal oxide interface(ref. 2). The ability of the TaN substrate to support a dielectric filmthat is resistant to the degrading effects of heat treatment can have asignificant impact on the performance of solid capacitors made from thisbi-layer system. The devices will have a low bias, frequency, andtemperature dependence of capacitance and probably be more stable duringaccelerated life testing. This latter prediction is based on the factthat life test failures are known to occur because of dielectricbreakdown associated with oxygen migration within and through the anodicoxide film. The more stable oxygen environment in anodic films grown arethe TaN substrate will make the system less susceptible to the long termdegradation mechanisms like ignition failures associated with oxygenmigration.

[0026] References

[0027] 1. Smyth, D. M. Shirn, G. A., and Tripp, T. B, J. Electrochem.Soc., 110, 1264-71(1963).

[0028] 2. Tripp, T. B., Shaw, M. D., and Cox, B, “The Effects ofNitrogen on the Properties of Anodic Oxide Films in Tantalum,”Proceedings of CARTS 99, in press.

[0029] 3. Werder. D. J., Kola, R. R. “Microstructure of Ta₂O₅ FilmsGrown by the Anodization of TaNx,” 323 Thin Solid Films 6-9 (1998).

Example 2

[0030] Six pounds of experimental tantalum powder prepared by thereduction of K₂TaF₇ by Na in known manner,¹ was presintered at 1320° C.for 30 minutes and deoxidized at 1000° C. for two hours using 2 percentmagnesium, in a known manner.²

[0031] Two pounds of such Ta powder were blended with 0.75% Mg andplaced in a tantalum foil lined stainless steel boat. The powder wasdeoxidized at 950° C. for two hours. The deoxidation was carried out toremove any oxygen associated with the tantalum that could interfere witha subsequent nitriding process.

[0032] The boat containing such mixture was placed under a slightlygreater than atmospheric argon pressure, and allowed to cool overnight.Next, the boat containing such mixture was heated to 680° C. as measuredby a thermocouple suspended inside the furnace tube over the boat. Thepressure was reduced to 600 mm Hg and nitrogen was introduced until thepressure was 760 mm Hg. The temperature was gradually increased and thepressure was maintained between 710 and 760 mm Hg by adding nitrogen asit was consumed by reaction with the tantalum. When the temperaturereached 720° C., the pressure was reduced to 460 mm Hg and the systembackfilled with nitrogen to 760 mm Hg. The temperature was increasedgradually to 760° C. and the pressure maintained in the range of 710-760mm Hg by adding nitrogen. The reaction was gentle as indicated by theslow rate of pressure drop. At this point, the pressure was reduced to250 mm Hg and backfilled with nitrogen to 760 mm Hg. The reaction rateincreased but still remained well controlled. The temperature wasallowed to increase to 830° C. and the pressure was maintained at710-760 mm Hg. Finally, the pressure was reduced to 0 mm Hg. Thetemperature was increased to give an inside temperature of 980° C. andthe environment was maintained at this temperature and under a slightpositive pressure of nitrogen for six hours.

[0033] After cooling to room temperature, the powder was passivated. Theresidual magnesium oxide and Mg₃N₂ were removed by leaching with diluteH₂SO₄ followed by rinsing to remove residual acid. The leached powderwas dried at 60° C.

[0034] The powder was analyzed for nitrogen and oxygen. The nitrogenconcentration was 38,600 ppm; this corresponds to the compound Ta₂N. Theoxygen concentration was 1650 ppm. The powder was tested for electricalproperties before and after heat treatment and after reformationfollowing heat treatment. The heat treatment conditions were 400° C. for30 minutes in air. The pellets were reformed at the formation voltagefor 30 minutes. The pellet preparation, formation and testing conditionsare summarized in Table VI, below. Four pellet sets corresponding to thethree sintering temperatures were formed together for each combinationof formation voltage and formation temperature. In general, theanodization process went smoothly under all conditions of sinteringtemperature, formation voltage and formation temperature.

[0035] Several four-volt, 225 μF solid capacitors were made using knownprocess condition developed for 30-50,000 CV/gm tantalum capacitors. Thepowder was pressed at 4.5 gm/cm³ and sintered at 1600° C. for twentyminutes. The pellet mass was 0.14 gm. No difficulties were encounteredin processing. The capacitors were tested as shown in Table I. None ofthe finished capacitors failed. The accelerated aging was carried out at85° C. for 240 hours with an applied voltage of 6.1V. All of thecapacitors survived the life testing.

[0036] The four pellet averaged capacitance values as a function ofsintering temperature, formation voltage and formation temperature afterformation, after heat treatment and after reformation are summarized inTable VII, below. The capacitance ranged from a high of 38,000 CV/gm atthe 1400°/60°/16V condition to a low of 18,500 CV/gm at the1600°/90°/80V condition. The capacitance dropped from 2 to 7 percentafter heat treatment depending upon the formation voltage. Thecapacitance drop was highest for the pellets formed to 16V. In general,the capacitance drop was higher for the pellets formed at 90° C.relative to those formed at 60° C. There was little further change incapacitance after the pellets were reformed.

[0037] The four pellet averaged leakages are collected in Table VIII,below. They ranged from a high of about 5.72 nA/μF·V to a low of 0.36na/μF·V after formation and 7.5 to 0.16 nA/μF·V after reformation. Ingeneral, the pellets formed at 90° C. had lower leakage than thoseformed at 60° C. The leakage increased significantly after heattreatment but in each case returned to a value close to theafter-formation value for the pellet when the pellets were anodicallyreformed.

[0038] The bias dependence of capacitance after formation, heattreatment and reformation are given in Table IX, below. The biasdependence was calculated as the percent change in capacitance with anapplied bias of 50 percent of the formation voltage relative to thecapacitance without bias. When the substrate is tantalum, the capacitorhas a negative bias dependence of capacitance of 8-10 percent after heattreatment. In the present case (Ta₂N substrate), there was no biasdependence (or at most a small positive bias dependence) of capacitanceafter heat treatment. This is strong evidence that a conductivityprofile associated with an oxygen deficiency profile does not exist inthe anodic film. This property can considerably simplify the process formaking solid capacitors and should give devices that are more stableunder high thermal stress conditions.

[0039] Table X, below summarizes the results for solid capacitance. Itwas possible to make very good 4 volt solid capacitors from the powder.The devices performed well during accelerated life testing. TABLE VISummary of Pellet Preparation, Formation and Testing ConditionsCondition Value(s) Pellet Mass 0.14 gm Press Density 5.0 g/cm³ SinteringTemperature 1400°, 1500°, 1600° C. Sintering Time 20 minutes FormationTemperature 60° C., 90° C. Formation Voltage 16, 40, 80 V FormationCurrent 100 mA/gm Hold Time 2 hours Formation Electrolyte 0.1 V/V %HD₃PO₄ DCL Test Voltage 70% V_(f) DCLL Soak Time 5 minutes Maximum BiasVoltage 50% V_(f)

[0040] TABLE VII Summary of Capacitance Results (μF · V · gram)Sintering Temperature Form. 1400 1500 1600 Voltage AF AHT AR AF AHT ARAF AHT AR 60° C. Formation 16 38,000 35,700 35,500 35,000 32,900 33,30027,300 25,500 2 40 34,700 33,500 36,300 31,700 30,700 33,600 25,80024,900 2 80 27,900 27,400 27,200 26,400 25,600 25,500 21,500 21,500 290° C. Formation 16 34,200 31,300 23,200 31,700 29,300 32,600 25,00023,300 2 40 32,200 30,000 22,500 29,200 28,300 25,300 23,300 22,600 2 8034,600 30,200 22,500 19,900 19,800 20,100 18,900 18,600 1

[0041] TABLE VIII Summary of Leakage Results (nA/μF · V) SinteringTemperature Form. 1400 1500 1600 Voltage AF AHT AR AF AHT AR AF AHT AR60° C. Formation 16 0.92 10.71 0.57 0.83 4.02 0.73 0.36 1.02 40 2.912.33 1.48 2.94 2.37 2.77 0.76 1.82 80 2.76 4.44 3.41 3.63 9.20 7.50 2.718.61 90° C. Formation 16 0.57 3.05 0.27 0.36 1.23 0.24 0.13 0.73 40 0.751.55 0.38 1.44 2.97 0.63 0.47 2.09 80 2.21 4.69 0.77 5.72 11.37  1.833.49 9.93

[0042] TABLE IX Summary of Bias Dependence of Capacitance Results(Percent) Sintering Temperature Form. 1400 1500 1600 Voltage AF AHT ARAF AHT AR AF AHT AR 60° C. Formation 16 −0.71 4.70 3.29 0.54 2.60 4.02−1.01 1.71 40 0.79 0.00 −0.34 0.57 −0.05 −0.41 −0.43 0.01 80 0.78 1.090.97 0.88 1.25 0.87 0.28 1.03 90° C. Formation 16 −1.22 4.01 2.47 −0.603.90 3.79 2.47 3.79 40 0.38 −0.10 1.260 0.36 −0.40 1.41 −0.42 −0.02 80−0.66 1.57 1.34 −0.57 0.70 0.78 −0.57 1.24

[0043] TABLE X Summary of Solid Capacitor Results Wet Wet Cap Solid DCLSolid Cap DCL (na/μF · V) uF · V/gram (na/μF · V) Cap Rec Yield 0.20526,200 0.310 22,310 85% 100% Life Test DCL (nA/μF · V) 0 Hours @ 6.1 V240 hours @ 6.1 V # 25° 85° 85° C. 25° C. Yield Shorts 0.310 2.249 2.1050.279 100% 0

Example 3

[0044] Niobium powder was made by reducing Nb₂O₅ with magnesium. Theresulting powder's properties are summarized in TableXI. TABLE XIProperty Value Fisher Avg. Part. Diam., FAPD (μ) 1.4 Scott Bulk Dens.,SBD (g/in³) 16.2 Surface Area (cm²/g) 2.3 Carbon 154 Chromium 34 Iron 47Nickel 74 Nitrogen 2880 Oxygen 9900 Silicon 30 Sulfur 13

[0045] One kilogram of the powder was blended with 22 gm of magnesiumand placed in a niobium foil lined stainless steel tray. The tray andits contents were put in a laboratory tube furnace. The tube was flushedwith argon. The temperature of the furnace was raised to 950° C. and thesystem maintained at this temperature for 2 hours under an argonatmosphere to deoxidize the powder. The furnace was allowed to coolovernight while maintaining a slight argon flow through the tube. Next,the temperature was increased to 680° C. as measured with a thermocoupleinside the tube suspended over the sample and the pressure was reducedto 610 mm Hg. Using nitrogen, the pressure was increased to 760 mm Hg.Over the next thirty minutes, the temperature was increased toapproximately 750° and the pressure was maintained between 660 and 760mm Hg by adding nitrogen gas to replace the nitrogen consumed in thereactor. At this point, the pressure was reduced to 460 mm Hg and thenincreased to 760 mm Hg using nitrogen. A significant increase in thereaction rate occurred as indicated by the rate of pressure drop and anincrease in the temperature. One hour after the start of the process,the temperature had reached 900° C. and the reaction was proceeding at amoderate rate as indicated by the rate of pressure decrease. Thepressure was reduced to 250 mm Hg and then the system was returned to760 mm Hg with nitrogen. A rapid pressure drop followed and atemperature increase occurred. The pressure was maintained at 760 mm Hgusing nitrogen additions for the next 15 minutes as the temperatureincreased to 960° C. Following complete evacuation of the tube, nitrogenwas added to bring the pressure to 760 mm Hg. Very little pressure dropfollowed indicating the reaction was essentially complete. Finally, thepressure was increased to 810 mm Hg and the system was maintained at1000° for six hours.

[0046] After cooling to room temperature, the powder was passivated bycontrolled exposure to aid. The powder was then leached with dilutesulfuric acid to remove MgO, Mg₂N₃ and residual Mg, and then with highpurity water to remove traces of acid. The oxygen and nitrogen contentof the powder were measured using a LECO 436-oxygen/nitrogen analyzer.The nitrogen concentration was 151,000 ppm corresponding to the compoundNbN without excess nitrogen. The oxygen content was 4300 ppm.

[0047] The powder was fabricated to anodes, anodized and tested in a wetelectrolyte for electrical properties before and after heat treatmentand after reformation at the formation voltage for 30 minutes. Thepellet preparation, formation, and testing conditions are summarized inTable XII. TABLE XII Condition Value(s) Pellet Mass 0.14 gm PressDensity 3.5 g/cm³ Sintering Temperature(s) 1500°, 1600°, 1700° C.Sintering Time 20 minutes Formation Temperature(s) 60° C., 90° C.Formation Voltage(s) Vf 16, 40 Formation Current 100 mA/gm Hold Time 2hours Formation Electrolyte 0.1 V/V % H₃PO₄ DCL Test Voltage 70% V_(f)DCL Soak Time 5 minutes Maxim. Bias Voltage 50% V_(f)

[0048] The pellets were heat-treated in air for 30 minutes at 400° C.Four pellet sets corresponding to the three sintering temperatures wereformed together for each combination of formation voltage and formationtemperature. It was not possible to anodize pellets sintered at 1400° C.and pellets could not be anodized to 80 volts at any of the sinteringtemperatures

[0049] The four pellet averaged capacitance values (in μF·V/gram) as afunction of pellet sintering temperature, formation voltage andformation temperature, after the steps of formation (AF), heat treatment(AHT) and reformation (AR) are given in Table XIII. TABLE XIII SinteringTemperature (° C.) Form'n 1500 1600 1700 Voltage AF AHT AR AF AHT AR AFAHT AR 60° Formation 16 41,000 38,600 37,900 37,700 35,800 35,400 26,40025,000 24,700 40 40,900 40,700 38,600 37,400 36,300 35,800 26,800 26,70025,600 90° Formation 16 37,000 32,100 30,300 34,400 30,400 29,200 24,80021,000 19,300 40 41,700 35,900 36,800 39,400 32,900 33,200 28,000 24,40024,100

[0050] The capacitance ranged from a low 19,300 μF·V/gm at the1700°/90°/16V condition and a high 41,700 μF·V/gm at the 1500°/90°/40Vcondition. There was an increase in capacitance when the formationvoltage was increased from 16 to 40V. A similar behavior occurs withniobium when the anodization voltage increases. This is contrary to thebehavior of anodes made with Ta substrates where capacitance decreasesas the formation voltage increases. The capacitance decreased from 0.5to 16% after heat treatment depending upon the formation voltage andformation temperature. The capacitance change was higher for the pelletsanodized at 90° C.

[0051] The leakage results (in nA/μF·V) are summarized in Table XIV.TABLE XIV Sintering Temperature (° C.) Form'n 1500 1600 1700 Voltage AFAHT AR AF AHT AR AF AHT AR 60° Formation 16 0.60 11.31 0.58 0.29 0.500.26 0.24 0.65 0.27 40 0.20 1.06 0.25 0.12 0.77 0.14 0.08 0.36 0.12 90°Formation 16 0.93 2.75 0.93 0.46 1.06 0.50 0.59 0.63 0.22 40 0.27 1.750.42 0.09 0.57 0.09 0.07 0.48 0.12

[0052] The highest leakage after formation was 0.60 nA/μF·V and thelowest was 0.09 nA/μF·V. In general, the heat treatment/reformationprocess lowered the leakage. Overall, pellets with the lowest leakageswere anodized at 60° C. to 40 volts.

[0053] Table XV, below, shows (as a %) the bias dependence ofcapacitance as a function of sintering temperature, formation voltage,and formation temperature. TABLE XV Sintering Temperature (° C.) Form.1500 1600 1700 Voltage AF AHT AR AF AHT AR AF AHT AR 60° Formation 16−0.96 −0.11 −0.20 −1.07 −0.14 −0.20 −2.06 −0.44 −0.22 40 −2.34 −0.11−0.28 −3.37 −0.10 −0.25 −8.61 −0.41 −0.31 90° Formation 16 −6.80 −4.56−0.28 −6.43 −1.79 −0.17 −5.42 −3.52 −0.35 40 −14.0 −2.70 −6.03 −16.3−0.76 −4.11 −15.2 −0.79 −3.15

[0054] The after formation pellets had a relatively large biasdependence of capacitance at the 90°/40V condition. This bias dependenceoccurs with the application of a small (˜1.5V) bias and littleadditional bias dependence occurs as the applied bias increases. This issimilar to the behavior for anodized niobium. In general, the biasdependence of capacitance was low after the pellets were heat-treated.Interestingly, the high bias dependence after anodization disappearsafter heat treatment. The insensitivity of the system to heat treatmentis striking given the fact that the anodic oxide film on niobium isseverely damaged when exposed to temperatures in the 170° C. range. Thelack of a bias dependence of capacitance after heat treatment isconsistent with a model that the niobium nitride/anodic oxide filminterface is more stable than the niobium metal/anodic oxide filminterface.

[0055] The foregoing example with included discussion/analysis showsthat it is possible to provide a niobium nitride powder and an anodizedsintered pellet form thereof to serve as an anode basis for capacitorswith low D.C. leakage and comply with the necessary criteria ofstability, reliability, cost, capacitance and ultimately fill the longextant gap between aluminum and tantalum and to some degree surpasstantalum systems. The new niobium nitride based system is suitable forsolid capacitors.

Example 4

[0056] A similar series of powder preparation and capacitor steps, as inExample 3, was carried out (with the added step of solid electrolyteimpregnation and conventional cathode application) to produce twenty4-volt solid electrolyte capacitor powders each comprising a sinteredporous (pores filled with manganese dioxide electrolyte made by thermaldecomposition of manganese nitrate) anode of niobium nitride powderswith anodic film as the dielectric. The anodes were made up as pressed3.5 gm blocks and sintered at 1700 deg. C. before oxidation (formation),impregnation and completion of capacitor processing. Tests ofcapacitance and leakage were conducted under wet and solid electrolyteenvironments, noting solid electrolyte presence capacitance as apercentage of wet electrolyte presence capacitance (capacitancerecovery, Cap. Rec.). Wet and solid D.C. leakage were also measured. Theaverage of twenty 4 volt capacitors was 26,400 CV/gm wet capacitance,24,100 dry, i.e. capacitance recovery of 91.5% D.C. leakage was 0.24nA/μF·V wet and 0.85 nA/μF·V solid. A 100% yield was obtained in theprocessing and test of the group of twenty. The solid electrolytesamples were life tested through an aging period of 40 hours.

[0057] Initially (at essentially 0 time elapsed) the average of leakagewas 0.85 nA/μF·V at 25 deg. C. and 6.63 nA/μF·V at 85 deg. C. After 40hours of aging at 85 deg. C. with an applied bias of 0.1 volts, theaverage of leakage at 85° C. was 5.93 nA/μF·V and at 85 deg. C. theaverage of leakage was 0.81 nA/μF·V. There were no shorts, voltagebreakdown, ignition or other runaway conditions among the 20 capacitors.All the 25 deg. C. items were below the established threshold of theindustry of 2.5 nA/μF·V, but high compared to TaN systems (studiedseparately and described in another co-pending provisional patentapplication), those skilled in the art will recognize that the leakagelevel can be substantially reduced for NbN systems by furtheroptimization of powder morphology, as well as sinter, formation andelectrolyte impregnation conditions.

[0058] The results demonstrate an improved process for making niobiumnitride (NbN) by removing the oxygen in the niobium feedstock bymagnesium deoxidation prior to nitriding. The nitride can be used as thesubstrate for making solid electrolyte capacitors with electricalquality comparable to that of devices made using tantalum as thesubstrate. The niobium nitride-anodic film interface isthermodynamically more stable than the niobium-anodic niobium oxideinterface (and compares well vs. a tantalum-anodic tantalum oxideinterface) as evidenced by the lack of a bias dependence of capacitanceafter heat treatment for the niobium nitride substrate system.

Example 5

[0059] Tantalum powder was made by conventional sodium reduction of apotassium fluotantalate precursor and powder leaching and sizing. Thepowder was presintered at 1320° for 30 minutes and deoxidized usingmagnesium to remove the excess oxygen introduced during agglomeration.The resulting powder's properties are summarized in Table XVI. TABLE XVIProperty Value Fisher Avg. Part. Dia., FAPD (μ) 2.0 Flow (Sec) 0.34Surface Area (cm²/g) 13,700 Scott Bulk Dens., SBD (gm/in³) 25.5 Carbon34 Calcium 2 Chromium 9 Copper 1 Iron 5 Potassium 13 Nitrogen 1,840Sodium 1 Nickel 11 Oxygen 4,130 Sulfur 10 Silicon 8

[0060] Approximately one kg of this powder was blended with 0.75 W/W %Mg and placed in the R&D tube furnace in a tantalum lined stainlesssteel tray. The furnace tube was evacuated, back-filled with argon andheated to 1000° C. This temperature was maintained for two hours todeoxidize the powder. The furnace was allowed to cool overnight.

[0061] The temperature was increased to 680° C. as measured with athermocouple inside the tube and suspended over the powder. The pressurein the tube was reduced to 610 mm Hg and the system back-filled withnitrogen until the pressure returned to atmospheric (760 mm Hg).Additional nitrogen was added to maintain an approximate atmosphericpressure as the nitrogen was consumed by reaction with the tantalum.Twenty minutes into the process, the pressure was reduced to 460 mm Hgand then increased to 760 mm Hg with nitrogen. At this point, thetemperature was 710° C. Again, the pressure was maintained at nearatmospheric with nitrogen additions as the temperature was increasedover the next 25 minutes to 850° C. The pressure was reduced to 250 mmHg and increased back to 760 mm Hg with nitrogen. While maintaining nearatmospheric pressure in the tube using nitrogen additions, thetemperature was increased to 1030° over a period of 50 minutes. Thepressure was then reduced to ˜0 mm Hg and the system filled withnitrogen to 810 mm Hg. The system was maintained at this temperature andpressure for five hours.

[0062] The furnace was allowed to cool to room temperature and thepowder was passivated using the high capacitance powder passivationcycle. The powder was leached with dilute H₂SO₄ solution to remove theMgO, Mg₂N₃ and any residual Mg. The residues of acid were removed byrinsing with high purity water. The powder was dried to 60° C.

[0063] The powder was analyzed for oxygen and nitrogen using a Leco 436oxygen/nitrogen analyzer. The oxygen concentration was 2840 ppm and thenitrogen content was 6.99% W/W % (75,200 ppm). This corresponds to thecompound TaN_(0.97).

[0064] The powders were tested for electrical properties before andafter heat treatment and after reformation following heat treatment. Theheat treatment conditions were 400° C. for 30 minutes in air. Thepellets were reformed at the formation voltage for 30 minutes. Thepellet preparation, formation and testing conditions are summarized inTable XVII. TABLE XVII Condition Value(s) Pellet Mass 0.14 gm PressDensity 5.0 g/cm³ Sintering Temperature 1500°, 1600°, 1700° C. SinteringTime 20 minutes Formation Temperature 60° C., 90° C. Formation Voltage16, 40, 80 Formation Current 100 mA/gm Hold Time 2 hours FormationElectrolyte 0.1 V/V % H₃PO₄ DCL Test Voltage 70% V_(f) DCL Soak Time 5minutes Maxim. Bias Voltage 50% V_(f)

[0065] Four pellet sets corresponding to the three sinteringtemperatures were formed together for each combination of formationvoltage and formation temperature. In general, the anodization processwent smoothly under all conditions of sintering temperature, formationvoltage and formation temperature. The four pellet averaged capacitancevalues (in μF·V/gram) as a function of pellet sintering temperature,formation voltage and formation temperature, after the steps offormation (AF), heat treatment (AHT) and reformation (AR) are given inTable XVIII. TABLE XVIII Sintering Temperature (° C.) Form'n 1500 16001700 Voltage AF AHT AR AF AHT AR AF AHT AR 60° Formation 16 35600 3330032900 28300 26500 26400 18100 17000 16800 40 32600 31800 31900 2660026300 26100 17600 17400 17400 80 26700 26900 26300 23000 22900 2250015900 15900 15800 90° Formation 16 32500 30800 30500 26300 24900 2470016600 15700 15600 40 28100 28100 27900 24200 23800 23800 15700 1560015700 80 20400 19300 15600 18000 17300 133300 13800 13600 13500

[0066] The capacitance ranged from a high of 35,600 μF·V/gm at the16V/60°/1500° condition to a low of 13,800 μF·V/gm at the 80V/90°/1700°condition. As with Ta₂N and NbN substrate capacitors, the capacitancedecreases after heat treatment and after reformation.

[0067] The leakage results (in nA/μF·V) are summarized in Table IXX.TABLE IXX Sintering Temperature (° C.) Form. 1500 1600 1700 Voltage AFAHT AR AF AHT AR AF AHT AR 60° Formation 16 1.40 9.89 2.25 0.33 0.540.31 0.31 0.70 0.28 40 2.06 3.79 1.09 1.72 2.11 0.63 1.23 2.89 0.67 803.92 3.55 3.41 4.00 3.38 3.39 3.89 3.70 3.42 90° Formation 16 0.39 16.10.42 0.16 0.36 0.12 0.18 0.55 0.16 40 0.80 1.92 0.28 0.79 1.59 0.71 0.461.61 0.17 80 5.72 12.87 4.84 8.15 13.47 6.21 6.07 13.73 5.07

[0068] The leakages increased as the formation voltage increased and thesintering temperature decreased. At the 16V and 40V formationconditions, the leakage decreased as the formation temperatureincreased. The reverse occurred at the 80V condition. In general, theleakages were lowest after reformation. The leakages for the TaN/anodicfilm capacitors are higher than the ones for Ta₂N/anodic film capacitorsand NbN/anodic film capacitors (production and testing described inseparate co-pending provisional applications) at the 16V and 40Vconditions. Except at the highest formation voltages, the leakages werein most cases acceptably low.

[0069] Table XXV, below, shows (as a %) the bias dependence ofcapacitance as a function of sintering temperature, formation voltage,and formation temperature. TABLE XXV Sintering Temperature Form. 15001600 1700 Voltage AF AHT AR AF AHT AR AF AHT AR 60° Formation 16 −0.31−0.05 −0.01 −0.29 −0.04 −0.09 −0.41 −0.04 −0.12 40 −0.18 −0.10 −0.07−0.20 −0.09 −0.07 −0.19 −0.11 −0.05 80 −0.10 −0.10 −0.11 −0.12 −0.12−0.13 −0.11 −0.10 −0.11 90° Formation 16 −0.32 −0.05 −0.02 −0.32 0.000.08 −0.38 −0.08 0.09 40 −0.16 −0.08 −0.08 −0.19 −0.08 −0.09 −0.15 −0.08−0.07 80 −0.10 −0.11 −0.06 −0.10 −0.11 −0.07 −0.10 −0.06 −0.05

[0070] It is seen that the tantalum nitride/anodic film system is veryinsensitive to heat treatment. TaN is better than Ta2N and NbN inresisting the effects of heat treatment.

[0071] The foregoing example with included discussion/analysis showsthat it is possible to provide a tantalum nitride powder and an anodizedsintered pellet form thereof to serve as an anode basis for capacitorswith low D.C. leakage and comply with the necessary criteria ofstability, reliability, cost, and capacitance. The system is suitablefor solid capacitors.

Example 6

[0072] A similar series of powder preparation and capacitor steps, as inExample 1, was carried out (with the added step of solid electrolyteimpregnation and conventional cathode application) to produce twenty4-volt solid electrolyte capacitor powders each comprising a sinteredporous (pores filled with manganese dioxide electrolyte made by thermaldecomposition of manganese nitrate) anode of niobium nitride powderswith niobium pentoxide as the dielectric. The anodes were made up aspressed 5.5 gm. blocks and sintered at 1600 deg. C. before oxidation(formation), impregnation and completion of capacitor processing. Testsof capacitance and leakage were conducted under wet and solidelectrolyte environments, noting solid electrolyte presence capacitanceas a percentage of wet electrolyte presence capacitance (capacitancerecovery, Cap. Rec.). Wet and solid D.C. leakage were also measured. Theaverage of twenty 4-volt capacitors was 27,500 CV/gm wet capacitance,24,600 dry, i.e. capacitance recovery of 89.6%. D.C. leakage was 0.26nA/μF·V wet and 0.14 nA/μF·V solid. A 100% yield was obtained in theprocessing and test of the group of twenty. The solid electrolytesamples were life tested through an aging period of 40 hours.

[0073] Initially (at essentially 0 time elapsed) the average of leakagewas 0.14 nA/μF·V at 25 deg. C. and 1.29 nA/μF·V at 85 deg. C. After 40hours of aging at 85 deg. C. with an applied bias of 6.1V, the averageof leakage was 1.44 nA/μF·V and at 25 deg. C. the average of leakage was0.18 nA/μF·V. There were no shorts, voltage breakdown, ignition or otherrunaway conditions among the 40 capacitors. All the. items were belowthe established threshold of the industry of 2.5 nA/μF·V.

[0074] The results demonstrate an improved process for making tantalumnitride (TaN) by removing the oxygen in the tantalum feedstock bymagnesium deoxidation prior to nitriding. The tantalum nitride can beused as the substrate for making solid electrolyte capacitors withelectrical quality comparable to that of devices made using tantalum asthe substrate. The tantalum nitride-anodic tantalum oxide film interfaceis thermodynamically more stable than the tantalum-anodic tantalum oxideinterface as evidenced by the lack of a bias dependence of capacitanceafter heat treatment for the tantalum nitride substrate system.

Example 7

[0075] The appended published paper (Tripp, Creasi and Cox “TantalumNitride: A New Substrate For Solid Electrolyte Capacitors”), includingall its text, footnotes (and the publications and presentations definedthereby) and drawings is incorporated herein by reference as though setout at length herein. The disclosure overlaps in part with the precedingExamples but also includes additional information.

[0076] It will now be apparent to those skilled in the art that otherembodiments, improvements, details, and uses can be made consistent withthe letter and spirit of the foregoing disclosure and within the scopeof this patent, which is limited only by the following claims, construedin accordance with the patent law, including the doctrine ofequivalents.

What is claimed is:
 1. A refractory metal nitride powder suitable as asubstrate for electrolytic capacitors and comprising a refractory metalnitride powder selected from and characterized in an ability when testedor otherwise subjected to capacitor anode sintering and formation andelectrolytic capacitor production and life test conditions by display ofan oxide of Ta, Nb interface with the substrate powder that issubstantially insensitive to heating of such test conditions and reducedbias and frequency dependence compared to un-nitrided analogs.
 2. Thepowder of claim 1 wherein the refractory metal nitrides are selectedfrom the group consisting of substantially (atomic basis) TaN, Ta₂N,NbN.
 3. An electrolytic capacitor anode comprising the power of claim 2.4. A wet electrolytic capacitor comprising the anode of claim
 3. 5. Asolid electrolyte capacitor comprising the anode of claim 3.